1. Field of the Invention
This invention relates generally to hoses and in particular to conductive hoses.
2. Description of the Prior Art
Given the well known fact that water and electricity do not mix, firefighters who find themselves in a position where they have to use water on an electrical fire, either by choice or by accident, are in very real danger of sustaining a severe if not lethal electrical shock using present equipment.
Although there are alternate methods to fight electrical fires, such as CO.sub.2, and HALON, none is as effective or as readily available as water. Yet even 120 volts, which is the most common voltage aboard ship or shore installations, can be lethal. A firefighter entering a smoke-filled room or compartment may not even know he is near or in contact with deadly electrical shock hazard.
Research conducted by Underwriters Laboratories, Inc. and cited in the Fire Chief's Handbook, indicates that the maximum continuous current to which an individual can be safely subjected is five milliamps (5 mA) and that there is a definite relationship between the length of exposure to electric shock and the effect of that shock. Many authorities consider that a current of 50 mA passing through the human body is the approximate lower limit likely to cause fatalities. In tests conducted at Purdue University and referenced in the Handbook, the effects of various 60 cycle currents are estimated where the resistance of the human body was assumed to be 5,000 ohms. A current of 1 mA will just be felt; 4 to 10 mA will cause a sense of pain; 30 mA may cause unconsciousness; and a current of 100 mA may be fatal.
A study by the National Bureau of Standards relating to ground fault circuit interrupters in buildings for protection against hazardous shock is in general agreement with the above results. The NBS study states that the resistivity of the skin varies with individuals. When dry, it may be as high as 100,000 to 300,000 ohms/cm, but when wet or broken by a cut, the resistance may be as low as 1% of this value. NBS further states that a value of 500 ohms is considered to be the minimum resistance of the human body between hands or between other major extremities such as hand and foot. This value is frequently used in estimating shock hazard currents in industrial accidents.
The effects of various levels of current on the human body vary greatly. For example, the level at which alternating current stimulates the nerves, indicated by a slight tingling sensation, is termed the "perception current". The mean perception current value for men is 1.1 mA at 60 Hz and the mean value for women is 0.7 mA at 60 Hz.
"Reaction currents" are those currents equal to or slightly greater than perception currents that could produce an involuntary reaction, while a "let-go current" is the maximum current a person can endure and still release the conductor by voluntary muscular control. The maximum uninterrupted, reasonably safe let-go currents are 9 mA for men and 6 mA for women.
Currents at slightly above let-go levels are currents at or a little above those at which a person can "let-go" of a conductor, but below currents causing a ventricular fibrillation (stoppage of heart action) and may contract chest muscles and stop breathing during the period of shock. Normal breathing may resume when the current is interrupted. However, with prolonged application of current, collapse, asphyxia, unconsciousness, and even death may occur in a matter of minutes.
Finally, the deadliest currents are currents causing ventricular fibrillation. The human heart rarely recovers from ventricular fibrillation. Experiments which cause stoppage of heart action and blood circulation cannot be conducted on man. However, data from animal experiments indicates that ventricular fibrillation in normal adults is unlikely if shock intensity is less than 116/T mA, where T is in seconds.